Image flipping can be processed following different code paths in bitmap.cxx.
Has been verified that it was broken in Linux Mint (with cinnamon) because
there a buggy code path was chosen which was not fixed
in commit 3119440a in master.
This commit solves the problem for this code path

Change-Id: I22257e70761ca5469c0424b5f9925681cfd4e2e3
Reviewed-on: https://gerrit.libreoffice.org/22753Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>